i've had an MBX6, an XRAY 808, and currently have an MP9 as well as a Durango, so I can give you a little firsthand feedback.
MBX6: awesome car, you can really overdrive it and depending on your setup, tends to be very forgiving, while still being nimble. Tends to exhibit a loose rear end, so take that into consideration with regards to your driving style. Front arms tend to be a wee bit fragile, but otherwise very robust.
808: depending on setup, exhibits Losi-style nervousness more often than not. If you're a very technical driver you may be able to get good lap times out of it, if you like to overdrive your car, you will not. Very solid machine, not prone to breakage, but you gotta watch those drive-line bearings! Prices on spares are a bit
Parts availability can also be an issue.
MP9: doesn't look like much, but boy can she dance! Tends to be very sensitive to setup, but performs way better than I could have thought possible, jumps extremely well too. Just like the MOOG, parts are plentiful, good parts cost to value ratio also. Might be a wee bit more fragile than the first two, but shouldn't be an issue unless you plan to use the car as a lawn dart
